1. Field of the Invention
The present invention relates to low maintenance scrubbers for the removal of water soluble and/or alkaline gases. The invention further relates to high efficiency acid scrubbers for the removal of hazardous components from an exhaust gas stream. The invention also relates to the removal of alkaline gases by reaction with an organic acid. The invention still further relates to methods of removing hazardous gases by their reaction with non-hazardous materials.
2. Background of the Related Art
In numerous chemical processes, waste gases pass into an exhaust stream. Many waste gas components, including by-products and excess reactants, are toxic and/or hazardous, and must be removed before the remaining exhaust is released into the environment. Two examples of hazardous gases which enter an exhaust stream are ammonia (NH.sub.3) and hydrochloric acid (HCl). These gases are both used for the hydride vapor-phase epitaxy (HVPE) growth of gallium nitride (GaN) and related semiconductors. Typically the ratio of ammonia (NH.sub.3) and hydrochloric acid (HCl) is very larger; on the order of NH.sub.3 /HCl=500. Further, only a fraction of the total quantity of each of these gases used during HVPE actually participate in a reaction, with the bulk passing into the exhaust stream. Because these waste gases are toxic and corrosive, they cannot be released into the atmosphere, and therefore must be separated from the non-toxic components of the exhaust stream and because ammonia (NH.sub.3) is the largest component of the waste gas efficient removal of ammonia (NH.sub.3) is one of the most crucial features of a scrubber system.
Fortunately, both ammonia and HCl are highly soluble in water. Thus, a simple method for removing them is to pass the exhaust stream through a volume of water. However, as the water becomes saturated with a waste gas, the efficiency with which the gas is absorbed decreases, allowing more and more of the waste to pass through the water over time. Saturation of a volume of water with dissolved waste gases can occur relatively rapidly, so that the water must be replaced frequently. A considerable amount of maintenance is therefore required, and the overall efficiency of such a process for waste gas removal is low.
Prior art methods for improving the removal of ammonia gas from an exhaust stream have involved the addition of inorganic acids, such as hydrochloric acid or nitric acid, to the water. This greatly increases the quantity of alkaline gases that can be absorbed, since the dissolved gas will react with the acids to produce inert salts. In order to provide high solubility of alkaline gases, high concentrations of acid are needed.
However, concentrated inorganic acids are hazardous to personnel, as well as being highly corrosive to system components and other equipment. In addition, hazardous by-products which require costly and difficult disposal, are produced. Furthermore, there is an inherent disadvantage in using mineral acids in that the quantity of these acids that can be added to the water is limited. As a result, relatively frequent replacement of the mineral acid/water mixture is required, leading to overall inefficiency of the scrubbing process.
However, solid acids, both inorganic and organic, including various carboxylic acids, have been used for the removal of gases from air. For example, U.S. Pat. No. 1,586,327 to Perrott, et al. discloses the purification of air to render the air suitable for breathing by providing a filter material composed of a granular mixture of an inert filler and an acid. Such a filter material may be packed into a canister of a gas mask for removing ammonia from air to be inhaled.
U.S. Pat. No. 5,626,820 to Kinkead, et al. discloses a chemical air filter for use in the air handling system of a clean room. The filter includes a pleated web of non-woven fibrous material, and having activated carbon particles distributed throughout the web. In one example of the '820, the use of citric acid in combination with a charcoal filter is disclosed.